The latest hydrocarbon production methods require that the production section of the well has a maximum possible length in the oil-bearing stratum. Since most oil-bearing production zones are substantially horizontal, this results in the final section of the well becoming appropriately horizontal. Although the general location of an oil-bearing stratum may be known prior to the drilling of a production well to tap the oil-bearing stratum, the position (in all dimensions) of the production zone is not initially known with sufficient accuracy to ensure that the well can be bored directly to the production zone. Accordingly, geological formation data are collected as the well is drilled, and the collected data are suitably analysed to derive the exact direction (in all three dimensions) along which the well is to be extended, particularly to ensure that the final (and usually horizontal) section of the well is in the best position for the recovery of oil. The procedure is known as “geosteering”.
Geological formation data are commonly gathered by gamma logging, i.e. by a procedure in which the intensity of detected gamma radiation is utilised to deduce geological properties. (While the source of gamma radiation may be naturally occurring radioisotopes more or less distributed throughout surrounding geological formations, a more usual source of gamma radiation is a manufactured gamma source (e.g., a compact mass of cobalt-60) emplaced at a fixed or controllably variable depth in an adjacent well such that the gamma source radiates through the geological formations between the gamma source and a gamma detector in the production well being drilled).
In order to geosteer, directional logging is necessary. For example, the intensity of detected gamma radiation above the bore of the well being drilled may be compared with the intensity of detected gamma radiation below the bore in order to decide the direction and extent by which to deviate the inclination of the next section of well to be drilled.
A gamma radiation detector typically comprises an assembly of a gamma-sensitive crystal (which emits a visible photon in response to the impact of a gamma photon), a photomultiplier (which outputs an electrical pulse count proportional to the light output of the gamma-sensitive crystal which, in turn, is proportional to the intensity of incident gamma radiation), and a pulse counter to accumulate a count, over a fixed interval, of electrical pulses from the photomultiplier.
The gamma radiation detector can be made directionally sensitive by surrounding the gamma-sensitive crystal with a gamma radiation shield (e.g., a tungsten shroud), the shield having an aperture or window through which gamma radiation can reach the gamma-sensitive crystal but only from one direction.
In order to carry out directional gamma logging of the well, it is necessary to orient the shield window to a selected angle with respect to a notional vertical plane through the well bore, and obtain a series of gamma intensity readings at various such angles, thereby to obtain a polar survey of geological formations surrounding the location of the detector.
In prior art well-drilling operations, the gamma radiation detector was incorporated into a bottom-hole drilling assembly. Directional gamma logging required that normal rotation of the drill string had to be stopped, and the drill string manipulated to orient the window to the required series of angles. The prior art directional logging procedure was therefore time-consuming, and prevented drilling during logging. (Transmission to the surface of logging data was also time-consuming, being usually undertaken by inducing pressure pulses in the drilling mud).
There is therefore a requirement for a means of conducting well logging operations such as gamma logging during drilling.
As will be discussed below, gamma logging during drilling requires the establishment of the angular orientation of a downhole assembly about the borehole axis. There are other situations in which knowledge of this angular orientation is desirable, for example in operation of the controllable stabiliser described in EP-A-1024245. The present invention aims to provide a convenient means of doing so.